Network Working Group D. Katz
Internet Draft Juniper Networks
D. Ward
Cisco Systems
Expires: September, 2005 March, 2005
BFD for Multihop Pathsdraft-ietf-bfd-multihop-02.txt
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Copyright (C) The Internet Society (2005). All Rights Reserved.
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Internet Draft BFD for Multihop Paths March, 2005
Abstract
This document describes the use of the Bidirectional Forwarding
Detection protocol (BFD) over multihop paths, including
unidirectional links.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [KEYWORDS].
1. Introduction
The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a
method for liveness detection of arbitrary paths between systems.
The BFD one-hop specification [BFD-1HOP] describes how to use BFD
across single hops of IPv4 and IPv6.
BFD can also be useful on arbitrary paths between systems, which may
span multiple network hops and follow unpredictable paths.
Furthermore, a pair of systems may have multiple paths between them
that may overlap. This document describes methods for using BFD in
such scenarios.
2. Issues
There are two primary issues in the use of BFD for multihop paths.
The first is security and spoofing; [BFD-1HOP] describes a
lightweight method of avoiding spoofing by requiring a TTL/hop limit
of 255 on both transmit and receive, but this obviously does not work
across multiple hops. The utilization of BFD authentication
addresses this issue.
The more subtle issue is that of demultiplexing multiple BFD sessions
between the same pair of systems to the proper BFD session. In
particular, the first BFD packet received for a session may carry a
Your Discriminator value of zero, resulting in ambiguity as to which
session the packet should be associated. Once the discriminator
values have been exchanged, all further packets are demultiplexed to
the proper BFD session solely by the contents of the Your
Discriminator field.
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Internet Draft BFD for Multihop Paths March, 2005
[BFD-1HOP] addresses this by requiring that multiple sessions
traverse independent physical or logical links--the first packet is
demultiplexed based on the link over which it was received. In the
more general case, this scheme cannot work, as two paths over which
BFD is running may overlap to an arbitrary degree (including the
first and/or last hop.)
3. Demultiplexing Packets
There are a number of possibilities for addressing the demultiplexing
issue which may be used, depending on the application.
3.1. Totally Arbitrary Paths
It may be desired to use BFD for liveness detection over paths for
which no part of the route is known (or if known, may not be stable.)
A straightforward approach to this problem is to limit BFD deployment
to a single session between a source/destination address pair.
Multiple sessions between the same pair of systems must have at least
one endpoint address distinct from one another.
In this scenario, the initial packet is demultiplexed to the
appropriate BFD session based on the source/destination address pair
when Your Discriminator is set to zero.
This approach is appropriate for general connectivity detection
between systems over routed paths, and is also useful for OSPF
Virtual Links [OSPFv2] [OSPFv3].
3.2. Out-of-band Discriminator Signalling
Another approach to the demultiplexing problem is to signal the
discriminator values in each direction through an out-of-band
mechanism prior to establishing the BFD session. Once learned, the
discriminators are sent as usual in the BFD Control packets; no
packets with Your Discriminator set to zero are ever sent. This
method is used by the BFD MPLS specification [BFD-MPLS].
This approach is advantageous because it allows BFD to be directed by
other system components that have knowledge of the paths in use, and
from BFD's perspective it is very simple.
The disadvantage is that it requires at least some level of BFD-
specific knowledge in parts of the system outside of BFD.
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Internet Draft BFD for Multihop Paths March, 20053.3. Unidirectional Links
Unidirectional links are classified as multihop paths because the
return path (which must exist at some level in order to make the link
useful) may be arbitrary, and the return paths for BFD sessions
protecting parallel unidirectional links may overlap or even be
identical. (If two unidirection links, one in each direction, are to
carry a single BFD session, this can be done using the single-hop
approach.)
Either of the two methods outlined earlier may be used in the
Unidirectional link case, but a more general solution can be done
strictly within BFD and without addressing limitations.
The approach is similar to the one-hop specification, since the
unidirectional link is a single hop. Let's define the two systems as
the Unidirectional Sender and the Unidirectional Receiver. In this
approach the Unidirectional Sender MUST operate in the Active role
(as defined in the base BFD specification), and the Unidirectional
Receiver MUST operate in the Passive role.
In the Passive role, by definition, the Unidirectional Receiver does
not transmit any BFD Control packets until it learns the
discriminator value in use by the other system (upon receipt of the
first BFD Control packet.) The Unidirectional Receiver demultiplexes
the first packet to the proper BFD session based on the physical or
logical link over which was received. This allows the receiver to
learn the remote discriminator value, which it then echoes back to
the sender in its own (arbitrarily routed) BFD Control packet, after
which time all packets are demultiplexed solely by discriminator.
4. Authentication
By their nature, multihop paths expose BFD to spoofing.
Implementations of BFD SHOULD utilize authentication over multihop
paths to help mitigate denial-of-service attacks.
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Internet Draft BFD for Multihop Paths March, 2005
Changes from the previous draft
No changes were made other than updating references.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
This document expires in September, 2005.
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